The present invention relates to a method and to apparatus for measuring the geoid and/or imaging the earth by processing radio signals from satellites.
Conventionally, geoid measurement or earth imaging at microwave frequencies is performed using synthetic aperture radar (SAR) of the kind found on board certain observation satellites, such as the satellite ERS-1.
In particular, by applying interferometry to SAR images, it is possible to track seismic deformations of the earth, earthquakes, volcanic eruptions, etc. . . . .
For a description of satellite radar interferometry techniques and applications thereof to imaging the earth and measuring the geoid, reference may advantageously be made to the various publications below:
"Deflation of Mount Etna monitored by spaceborne radar interferometry", Didier Massonnet et al., Letters to Nature, Vol. 375, No. 6532, Jun. 15, 1995; PA1 "Radar interferometric mapping of deformation in the year after the Landers earthquake", Didier Massonnet et al., Letters to Nature, Vol. 369, May 19, 1994; PA1 "Satellite radar interferometric map of the coseismic deformation field of the M=6.1 Eureka Valley, Calif. earthquake of May 17, 1993", Didier Massonnet, Kurt L. Feigl, Geophysical Research Letters, Vol. 22, No. 12, pp. 1541-1544, Jun. 15, 1995; PA1 "The displacement field of the Landers earthquake mapped by radar interferometry", Didier Massonnet et al., Letters to Nature, Vol. 364, No. 6433, Jul. 8, 1993; PA1 "Discrimination of geophysical phenomena in satellite radar interferograms", Didier Massonnet, Kurt L. Feigl, Geophysical Research Letters, Vol. 22, No. 12, pp. 1537-1540, Jun. 15, 1995. PA1 "A passive reflectometry and interferometry system (Paris): application to ocean altimetry", Martin-Neira M, E.S.A. Journal, Vol. 17, No. 4, Jan. 1, 1993, pp. 331-355, PA1 the reference signal is a radio signal picked up directly from the satellite by means of an antenna disposed in the vicinity of the directional antenna; PA1 correlation processing is implemented between the reference signal and the reflected radio signal from the satellite as picked up by the directional antenna, together with a phase correction to take account of the displacement between two measurements of the satellite which transmits the picked-up signal; and PA1 the picked-up radio signal from the satellite is a GPS radio positioning signal or an SAR signal.
Reference may also be made to the following publication:
which describes a method of calculating altitude without making use of an image.
Geometers on the ground find it difficult to make use of satellite-borne SAR observation of the earth, given that in general they are not specialists in such techniques.